1. Field of the Invention
This invention relates to a power transmission device having the function of a torque limiter. More particularly, the invention relates to a power transmission device that is used when assembled into a car compressor.
2. Description of the Related Art
In a power transmission device for transmitting power to a compressor in the prior art, a coupling method of a hub having a damper attenuation mechanism made of a material such as rubber and a pulley has been conducted by a method shown in FIGS. 27A and 27B. In other words, when a pulley A is made of a metal such as iron, a screw thread is formed on the pulley A and the pulley A and a hub B are coupled by a bolt C, or the like (refer to Japanese Unexamined Patent Publication No. 2001-153152). Incidentally, symbol D represents a rubber damper.
In the case of a pulley A made of a material having a relatively low material strength, compared to metal, such as a resin, a metal E capable of screw meshing is coupled with the pulley A by a method such as insert molding, and then the bolt C, or the like, is used to thereby couple the hub B and the pulley A (refer to Japanese Unexamined Patent Publication No. 2003-56595).
However, a component such as a metal fitting for screwing with the bolt must be added in the coupling method shown in FIG. 27B and the production cost increases. In the case of a pulley made of a resin material, for example, the metal fitting must be disposed by means such as insert molding. In addition, the problem remains that the coupling strength between the metal fitting and the resin material remarkably drops owing to a change of the resin material with time.
A reinforcing material, such as glass fibers, is incorporated in the resin to improve the material strength in the case of the resin pulley. However, the orientation of the glass fibers cannot easily be made the desired orientation by insert molding of the metal fitting, so that a good coupling strength cannot be acquired around the metal fitting.
Therefore, the prior art technology employs the construction in which a convex portion B1 is formed around an outer ring of the hub B, a concave portion A1 is formed around the inner circumference of the resin pulley A and they are fitted to each other as shown in FIG. 27C (refer to Japanese Unexamined Patent Publication No. 2002-364667) so that the bolt or the like is not necessary when the hub B and the pulley A are fitted to each other.
According to this prior art technology, however, the convex portion of the outer ring and the concave portion of the pulley come into direct contact with each other due to excessive torque or a torque fluctuation, and the concave portion of the resin pulley having lower strength and wear resistance than the outer ring is abnormally worn out.